In friction stir welding, a rotating tool having a disk-shaped or cylindrical shoulder and a pin projecting axially from the shoulder is displaced transversally to its rotating axis along the workpiece consisting of one or two parts, the rotating pin penetrating into the workpiece. The rotation of the pin on and within the workpiece leads to a temperature increase and to a softening of the workpiece, wherein it is also known to stir the materials of the workplaces together in the solid state. Different methods and different friction stir welding tools are known. In the conventional friction stir welding in which two adjacent metal sheets having a gap there between are stir-welded together, a concave weld seam is produced via the set angle and the diameter of the rotating shoulder. The shoulder is for example a cylindrical rotating part the end face facing the workpiece of which serves as a workplace contact surface. In this case, the shoulder rotates together with the pin. Due to the gap between the two workpiece parts, filling material is missing upon welding such that in case of wide gaps, shriek holes and thus weaker weld seams may be produced. In an initially two-part workpiece for example, the pin which projects with respect to the workpiece contact surface is displaced along the gap between the workpieces and penetrates in both parts.
In addition to tools in which the pin and the shoulder rotate together, there also exist tools having non-rotating shoulders which have lower process forces than friction stir welding tools having a rotating shoulder. Due to the planar rest of the non-rotating shoulder on the workpiece surface, a seam surface of very high quality without the ripples typical of friction stir welding is produced. The fatigue properties of the connection are thus considerably improved, and the weld seam may be painted without further preliminary works so as to have the same qualify as a relied metal sheet. The ability to bridge the gap is however highly reduced by the plane rest of the non-rotating shoulder. To date, this requires high efforts concerning the seam preparation. The tolerances of the assembly parts have to be chosen very close. Furthermore, in these tools having an upright shoulder, material of the workpieces penetrates into the annular gap between the pin and the shoulder. A regular cleaning of the tool is then necessary to avoid a sticking of the pin and the shoulder. The term “non-rotating shoulder” of course does not exclude that the shoulder is rotated upon alignment of the tool or upon displacement along a curved path. The shoulder however does not relate interminably in one direction of rotation to make quite a lot of revolutions, rather, it rotates only about some angular degrees.
A further disadvantage arising from a too small volume of material is the generation of so-called worm holes or shrink holes. Worm holes or shrink holes can be produced in case of a missing material volume in the initial configuration by large gaps or in case of a one-part workplace by already existing shrink holes. Shrink holes or worm holes may further be formed in the region of the weld seam if material leaves the region of the weld seam by the temperature-related softening and plastic deformation of the material adjoining the weld seam. In the prior art, these drawbacks are to be eliminated by concepts which provide a supply of additional material, i.e. of material from outside of the workplace. Concerning this, there exist the following fundamental ideas.
A first idea provides a so-called hollow spindle having an integrally connected pin and a shoulder through which a channel is formed via which the additional material is supplied. An example thereof is document U.S. Pat. No. 6,543,871 B2. The problems here are the risk of a plugging of the supply channel between the pin and the spindle for the additional material and the provision of sufficient pressure for the material to be supplied since a considerable counterpressure is built up in the region of the workplace during friction welding.
An improvement of this method provides that a conveyor worm having a small diameter extends through the channel and thus through the shoulder and the pin and thus softens the additional material. This method is shown in documents US 2012 0279441 A1 and US 2012 079442 A1. A very fine and powdery additional material is required therefor to not overload the filigree worm.
A further method provides the application of a granular material onto the workpiece which is passed over by the shoulder and thus deformed and plasticized. Aluminum powder can be used here as granular material. The control of the sufficient quantity of granular material and the sufficient inclusion thereof in the already present material is however difficult in case of large set angles, there is furthermore the risk that the granular material is ejected from the so-called inlet gap.
A third concept provides the placement of a separate workpiece into the gap between the two parts that are to be welded together and the closure of the gap. This concept however requires the exact definition of the gap and the insertion of a workpiece having an appropriate cross-section. To realize this, the gap is brought to size in a previous work operation by a machining process, and a wire having a rectangular cross-section then for example inserted into the gap.
Finally, a consumable pin is also known which is axially shiftable with respect to the shoulder, itself also softened upon rotational motion and then also fills the gap itself. However, since the pin is softened itself with the process heat, the welding depth which can be obtained therewith is restricted. The welding length is limited by the volume of the consumable pin, and a very complex feed system is necessary to continuously feed the pin further axially during the welding process. Such a method is known from document US 2005 0045635 A1.
The object of the invention is to present a friction stir welding tool and a friction stir welding method ensuring an improved ability of bridging a gap and guaranteeing a high welding quality. According to one embodiment of the invention, the possibility is to be created to fill the end hole typical of friction stir welding in a simple manner during the process. The end hole is generated in that the tool is pulled out of the workpiece at the end of the weld seam and no material is available to close the volume occupied by the pin.